PETROGRAPHY AND DIAGENESIS OF THE MIDDLE CAMBRIAN LION MOUNTAIN SANDSTONE OF THE RILEY FORMATION, LLANO UPLIFT, BURNET COUNTY, TEXAS

In the Llano Uplift of Central Texas, Proterozoic basement rocks are unconformably onlapped by Paleozoic sedimentary strata. The oldest exposed sedimentary rocks are those of the middle to upper Cambrian Moore Hollow Group. The Riley Formation of the Moore Hollow Group consists of three members, in ascending order, the Hickory Sandstone, the Cap Mountain Limestone, and the Lion Mountain Sandstone. The Lion Mountain Sandstone member crops out in Burnet County, Texas, and its uppermost unit is readily identified by its distinctive dark-green color, the result of extensive glauconization. This glauconite-rich, fossiliferous, trough cross-stratified quartz sandstone has been interpreted as having accumulated in a shallow-water environment as part of a laterally migrating tidal inlet complex associated with a barrier bar system. The widespread presence of glauconite, which occurs predominantly as pellets, cement, and as a replacement of fossils, remains somewhat problematic, as glauconite is typically associated with deeper-water shelf and slope environments. Petrographic analysis quantified the composition of framework detrital grains and interstitial cements. Framework grains, dominantly monocrystalline quartz originally derived from the Proterozoic basement, were subsequently reworked by shallow-marine processes. As to be expected with a 500 million year old sandstone, diagenesis is complex, involving multiple precipitation and dissolution events.